This is just a little thing I was working on right now. Eventually this will be part of a real game. Eventually.
It's pretty obviously incomplete (there's no background, or loss condition, or menus, and it doesn't keep spawning things as you move away), but it's almost a start.
You can move the "player" (the one at the center of the screen) with either WASD or the arrow keys, add new bodies to the simulation with the numpad's plus key and reset it with the 'r' key.